LAS VEGAS — Having sex in the weightlessness of outer space is the stuff of urban legends and romantic fantasy — but experts say that there would be definite downsides as well.

Spacesickness, for instance. And the difficulty of choreographing intimacy. And the potential for sweat and other bodily fluids to, um, get in the way.

"The fantasy might be vastly superior to the reality," NASA physician Jim Logan said here Sunday at the Space Frontier Foundation's NewSpace 2006 conference. Nevertheless, Logan and others say the study of sex and other biological basics in outer space will be crucial to humanity's long-term push into the final frontier.

"Sex in space is not just a good idea, it's survival," said Vanna Bonta, a writer who blends romance with space travel and quantum physics in the novel "Flight."

Sex in the space environment has long been a source of rumor and speculation: Several years ago, one author claimed that NASA had conducted a study of sexual behavior during a space shuttle mission, sparking a quick round of denials. Today, NASA follows something of a "don't ask, don't tell" policy on the subject — leading Logan to stress that he was not representing the space agency at Sunday's panel discussion.

The subject is coming to the fore again now for several reasons — including next month's publication of a book by Laura Woodmansee titled "Sex in Space," as well as billionaire Robert Bigelow's plan to host research into animal propagation on his commercial space modules.

After all, sometime in the next decade Bigelow Aerospace envisions putting a hotel complex in orbit, "where people will probably be recreating and having sex," Bonta said.

Woodmansee said sex would be "the killer app of space tourism ... because every couple who goes up there, or threesome or whatever their personal choice is, is going to want to try this."

However, off-Earth romantics will have to cope with some practical challenges:

Sex in space would likely be "hotter and wetter" than on Earth, Bonta said, because in zero-G there is no natural convection to carry away body heat. Also, scientists have found that people tend to perspire more in microgravity. The moisture associated with sexual congress could pool as floating droplets.

The physics of zero-G make the mechanics of sex more complicated. Bonta said it was challenging even to kiss her husband during a zero-G simulation flight they took recently. "You actually have to struggle to connect and stay connected," she recalled. Partners would have to be anchored to the wall and/or to each other. To address that need, Bonta has come up with her own design for garments equipped with strategically placed Velcro strips and zippers.

Although zero-G could be a boon for saggy body parts, Bonta said males might notice a "slight decrease" in penis size due to the lower blood pressure that humans experience in microgravity.

Romantics will also need to guard against the type of motion sickness that space travelers often encounter, especially if they get too adventurous right off. "Save the acrobatics for post-play vs. foreplay," Bonta advised.

For all these reasons, Logan said spontaneous sex in space could be "a little underwhelming."

"It's a pretty messy environment, when you think about it," he said. "And for every action there is an equal and opposite reaction. However ... I can well imagine how compelling, inspiring, and quite frankly stimulating choreographed sex in zero-G might be in the hands of a skilled and talented cinematographer with appropriate lighting and music."

When the crowd tittered, Logan added, "I'm not kidding: Sex in zero-G is going to have to be more or less choreographed. Otherwise it's just going to be a wild flail."

Bonta suggested that a honeymoon space hotel could offer specially designed environments to enhance zero-G intimacy — for instance, "hydro rooms" filled with floating droplets of cool water or scented oil.

The issue of what happens after sex is, if anything, more crucial for those concerned about future generations of spacefarers. The animal studies conducted so far indicate that the "absence of gravity loading would cause all kinds of problems" for fetal development, Logan said.

For example, Russian studies with pregnant rats showed a 13 to 17 percent arrest in the development of nearly every area of the fetal skeleton in zero-G, he said. Logan also noted that the proper formation of neural connections — a process that continues even after birth — requires movement under gravity loading. Immune functions are also compromised in microgravity.

Logan isn't worried so much about the early weeks of pregnancy, but he said studies have shown that gravity should play a significant role for human fetuses after about 26 weeks of gestation.

"This has significant implications for the colonization of the solar system," he said. Multigenerational life might be impossible without at least some gravity.

How much gravity?
So how much gravity is enough? The one-sixth gravity of the moon, or the one-third gravity of Mars? So far, no one knows, Logan said.

"We still do not have an inkling of what the 'gravity prescription' is," he said. "Think of gravity as a medication. We don't know the dose, we don't know the frequency, and we don't know the side effects."

Cosmic radiation in the space environment is another worry surrounding fetal development in space — and Logan said there may be a synergistic relationship between radiation and the ill effects of zero-G on the fetus. The unknowns are of such great concern that, given the current state of our knowledge, pregnancy in space would be "very dangerous," he said.

The efficacy of oral contraceptives in space is also a subject of concern, Woodmansee said, particularly because studies involving other types of medications indicate that drugs aren't absorbed as readily in space as they are on Earth.

Beyond the romance, more research
Logan as well as Woodmansee called for more research into how biological processes work in reduced-gravity environments — not just in the near-weightlessness of the international space station, but also on research satellites that can reproduce one-third or one-sixth gravity. Only then can scientists figure out how much gravity will be required to keep space-dwelling romantics alive for the next generation.

"If you can't figure out how to 'bioneer,' you're not going anywhere," Logan said.

But if humanity can figure out how to live and reproduce in space, it would represent a giant leap — not only for lovers, but for evolution as well, Woodmansee said.

"Our children and our grandchildren, et cetera, will be space aliens," she said. "They will change how humans are. They will be different from us in some way. Maybe if we are really going to go into space, these are good adaptations, but they're going to be painful, I think, in any case. It's disturbing, but it's something that we need to think about if we are truly going to be a spacefaring civilization and settle the galaxy."